When I first started freediving I carried my knife on my leg and used my open heeled scuba fins. I lost two knives that were pulled off my leg after getting tangled in kelp, and constantly had kelp pulling at my fin buckles. I eventually moved my knife to my forearm and bought closed heeled freedive fins. I know everything's a trade off, but these two changes sure cut down on kelp snags.
My condolences to the family.
Ron.